1. Field of the Invention
The invention relates to novel paper coating slips. The invention further relates to the use of paper coating slips and to papers coated with said slips.
2. Description of the Background
Paper coating slips consist essentially of a pigment (usually white), a polymeric binder, and additives which exert the desired influence over Theological properties of the slip and surface properties of the coated paper. Such additives are frequently also referred to as cobinders. The function of the binder is to fix the pigments to the paper and ensure cohesion within the coating obtained.
Slip coating gives base papers a smooth, uniformly white surface. The slips also enhance the printability of the paper.
The coating of paper with slips is nowadays well known; see, for example, “The Essential Guide to Aqueous Coating of Paper and Board”, T. W. R. Dean (ed.), published by the Paper Industry Technical Association (PITA), 1997.
One of the most important objectives of the slip coating of paper is to increase the whiteness of the paper. At the same time, however, the surface of the paper must be sufficiently stable that it is not damaged during printing and that the printed image is not impaired.
A host of different measures may be taken to increase the whiteness of the coated paper. These measures include, for example, the use of a base paper of very high whiteness, which in turn can be obtained by using very white starting materials. Another measure is to select very white pigments for the paper coating slip.
These measures alone, however, are in the majority of cases unable to meet the modern-day requirements of users regarding the whiteness of the paper. It is for this reason that products known as whiteners (fluorescent or phosphorescent dyes) or optical brighteners are added to the coating slip. The brighteners are dyelike fluorescent dyes which absorb the shortwave ultraviolet light that is invisible to the human eye and emit it as longer-wave blue light, giving the human eye the impression of a higher whiteness, so that the whiteness is increased.
The use of the optical brighteners, however, only results in the desired outcome if the brighteners are present in an optimum structure, conformation, and distribution in the finished paper coating. To achieve this, polymeric compounds, which intensify the effect of the optical brightener and are referred to as activators or carriers, are added to the slip. An important function of the cobinders—mentioned at the outset—in colored coating slips is their brightener-activating effect. Suitable cobinders may include water-soluble polymers, e.g., polyvinyl alcohol, carboxymethylcellulose, anionic or nonionic degraded starches, casein, soy protein, and water-soluble styrene-acrylate copolymers (see, for example, K. P. Kreutzer, Grundprozesse der Papiererzeugung 2: Grenzflächenvorgänge beim Einsatz chemischer Hilfsmittel, H. -G. Völkel and R. Grenz (editors), PTS Munich, 2000, PTS manuscript: PTS-GPE—SE 2031-2).
However, it is not possible with every water-soluble polymer to activate the optical brightener; for example, with certain polysaccharides, e.g., dextran, or with anionic polyacrylamides, it is not possible to activate the optical brightener. On the contrary, it has long been known that water-soluble polymers containing cationic groups, especially amino or ammonium groups, such as polyamine/epichlorohydrin or polyamidoamine/epichlorohydrin resins, polyamines or polyethyleneimines, for example, not only do not boost the whitener but in fact cause its deactivation, so that the brightening is extinguished (K. P. Kreutzer, loc. cit., page 8-22).
DE-A 197 27 503 discloses paper coating slips which comprise binders containing N-vinylcarboxamide units. The activation of optical brighteners by additives, however, is not described.
It is an object of the present invention to provide paper coating slips having improved properties or leading to an improvement in the coated paper.